1. Field of the Invention
The present invention relates to a circuit for selecting the slew rates of output buffers in an integrated circuit device. More specifically, the present invention relates to a circuit for automatically selecting the slew rates of output buffers in a programmable logic device attached to a populated printed circuit board, such that the output buffers experience a selected amount of signal reflection.
2. Related Art
Digital integrated circuits (ICs) are typically mounted on a printed circuit board, thereby enabling these ICs to be coupled to other integrated circuits, which are also mounted on the printed circuit board. When the digital ICs drive signals out onto the printed circuit board, signal reflections may occur as a result of the impedances of the printed circuit board traces and/or the impedances of other ICs mounted on the printed circuit board. In general, signal reflection occurs when an output signal is driven from a source pin onto a signal path having a relatively low impedance. The low impedance results in the output signal reflecting from the intended destination back to the source pin. If the source pin is an input/output (I/O) pin which receives input signals in addition to providing output signals, the reflected signal may be erroneously interpreted to be a valid input signal.
Early determination of signal reflections on a printed circuit board is critical to successfully debug today's high-speed circuits. Finding unwanted signal reflections is one of the single most difficult tasks to accomplish and frequently results in printed circuit board re-design to eliminate undesired signal reflections.
Time domain reflectometry (TDR) is a conventional technique used to determine passive impedance by using voltage pulses and timing measurements (similar to the manner in which radar devices determine distance). Printed circuit board manufacturers often use TDR to determine the impedance of printed circuit board traces. System designers typically use this impedance information to determine the time delay of signals that are driven onto the printed circuit board. When known, this impedance information helps to determine signal behavior on the printed circuit board, and enables the signal behavior to be improved if necessary. However, the impedances of the printed circuit board traces are measured before there are any devices mounted on the printed circuit board. The presence of devices mounted on the printed circuit board can change the effective impedances of the printed circuit board traces. Sophisticated circuit simulators have been used to model the behavior of devices attached to printed circuit boards with well-known circuit parameters. However, this modeling is difficult and frequently inaccurate.
It would therefore be desirable to have a system which determines the presence of signal reflections on a populated printed circuit board, and automatically eliminates or adjusts detected signal reflections.